Jairus T. Hammond was born around 1839 in Massachusetts to Thomas Hammond and Permelia Reynolds. His father was a cordwainer who owned $50 of personal property in 1860. Hammond grew up and attended school in Warwick, Massachusetts, until the 1850s, when the family moved to Wendell, Massachusetts. By 1860, he was working as a laborer and mechanic.
He enlisted in the Union army on October 12, 1861, and mustered in as a private in Company K of the 26th Massachusetts Infantry six days later. The regiment spent most of the war in the Department of the Gulf. The army transferred them to the Army of the Shenandoah in July 1864, and they took part in the Shenandoah Valley campaign that fall. He mustered out in Savannah, Georgia, on August 26, 1865.
Hammond returned to Wendell after the war, and he married Mary Mahany around 1866. They had at least two children: Nina, born around 1866; and Harvey, born around 1869. By 1870, he was working in a piano shop. They moved to Orange, Massachusetts, sometime in the 1870s, and by 1880, Hammond was working as a carpenter. Mary died in the 1880s, and he married Margaret Barnes on November 30, 1890. He died of “cardiac rheumatism” in Athol, Massachusetts, on April 6, 1901.